Toxoplasmosis is among the most common parasitic diseases of man. Serosurveys suggest prevalence rates as high as 70-90% in many areas of both the developing and developed world. Between 10-45% of Americans become infected at some point in their lives. An infection in an individual with a competent immune system generally has minor or no symptoms. The infection tends to be self-limiting, with the individual's immune system controlling and eliminating most of the parasites. Some parasites remain in bradyzoite form following acute infection and will be present in cysts in the central nervous system and muscle throughout the remainder of the individual's life.
In contrast to the mild clinical symptoms of infection seen in a healthy individual with an intact immune system, subjects with weakened or otherwise compromised immune systems can have serious clinical effects from toxoplasma infection. In the fetus, toxoplasma infection can cause mental retardation, visual defects, and death. Toxoplasma infection can cause neurological damage, ocular lesions and death in adults with compromised immune systems, a group which includes for example individuals with HIV infection or patients undergoing immune-suppressive treatment for cancer.
Acute toxoplasmosis can be difficult to treat. Sulfadiazine/pyramethamine is a regimen of choice, although side effects serious enough to warrant discontinuation of treatment are common. The toxic and potentially teratogenic effects of this regimen make management of the pregnant woman particularly problematic. AIDS patients require lifelong suppressive therapy to prevent relapse, and a many as one third of the patients receiving suppressive sulfadiazine/pyrimethamine therapy cannot tolerate the adverse side effects. For those who can tolerate the drugs, relapse occurs frequently. Pyrimethamine/clindamycin is a useful alternative therapy in AIDS patients who suffer an unusually high frequency of side effects from sulfa drugs. Unfortunately, this alternative combination can also cause considerable toxicity and is less effective at preventing relapse. Prevention of transmission through vaccination would be preferable to treatment, particularly for pregnant women and the immunocompromised.